Aquinas's Proofs 1 - FIRST MOVER: Some things are in motion, anything moved is moved by another, and there can't be an infinite series of movers. So there must be a first mover (a mover that isn't itself moved by another). This is God. 2 - FIRST CAUSE: Some things are caused, anything caused is caused by another, and there can't be an infinite series of causes. So there must be a first cause (a cause that isn't itself caused by another). This is God. 3 - NECESSARY BEING: Every contingent being at some time fails to exist. So if everything were contingent, then at some time there would have been nothing -- and so there would be nothing now -- which is clearly false. So not everything is contingent. So there is a necessary being. This is God. 4 - GREATEST BEING: Some things are greater than others. Whatever is great to any degree gets its greatness from that which is the greatest. So there is a greatest being, which is the source of all greatness. This is God. 5 - INTELLIGENT DESIGNER: Many things in the world that lack intelligence act for an end. Whatever acts for an end must be directed by an intelligent being. So the world must have an intelligent designer. This is God.
This question will get you more response if you put it on the discussion board instead of posting it here. I've included the proofs above, and just off the top of my head, I would say that his arguments are cyclical and they seem paradoxical. However, on closer look, he is using much of what science uses today to prove certain laws. For instance, for every action there is an equal reaction. Aquinas says essentially the same thing in Proofs 1 and 2. Proof 3 also seems true via circular logic. There has never been "nothing" unless you believe in the Big Bang theory, but even so, since the "bang" there has never again been absolutely nothing. We have also assigned a hierarchy to living things in the universe. Is it not true that we consider humans to be above all other living things on earth? What about living things not on the earth? Are they above us? I think Aquinus agrees there are, and he defines this as God. In addition, Aquinas believes that there is an intelligent being who created all life. How else can you explain the complexities of the human body, the ecosystem, the solar system, and the way life works in general on this planet? Of course, Aquinas didn't live to see the harm human have caused to earth with all of our factories, pollution, and non-green living, either, although I doubt that would change his mind as to the existence of God.